those should be the correct fittings. when you are looking at the outside of the whole fitting, yeah the threaded side is bigger than the barb side right? mine is too. but look through the hole, it is the same all the way through. no reduction.
i mean, these measurements are pretty much universal/standardized, i don't think my 1/2" x 3/8" is any different than the one at your store.
why don't you just buy one, take it home, or do what i do, take the pump right into the store. then put it on, and see. take your waterblock to compare too. you will see they are the same size barbs.
maybe i should take some pictures and send it to you?
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